3Com Switch 8800 Advanced Software V5 Configuration Guide

MSTP Overview 101
After the comparison processes described in the table above, a spanning tree with
Switch A as the root bridge is stabilized, as shown in
Figure 27.
Figure 27 A spanning tree with Switch A as the root bridge
n
To facilitate description, the spanning tree computing process in this example is
simplified, while the actual process is more complicated.
2 The BPDU forwarding mechanism in STP
■ Upon network initiation, every switch regards itself as the root bridge,
generates configuration BPDUs with itself as the root, and sends the
configuration BPDUs at a regular interval of hello time.
■ If it is the root port that received the configuration BPDU and the received
configuration BPDU is superior to the configuration BPDU of the port, the
device will increase message age carried in the configuration BPDU by a certain
rule and start a timer to time the configuration BPDU while it sends out this
configuration BPDU through the designated port.
■ If the configuration BPDU received on the designated port is inferior to the
configuration BPDU of the local port, the port will immediately send out its
superior configuration BPDU in response.
■ If a path becomes faulty, the root port on this path will no longer receive new
configuration BPDUs and the old configuration BPDUs will be discarded due to
timeout. In this case, the device will generate a configuration BPDU with itself
as the root and send out the BPDU. This triggers a new spanning tree
computing process so that a new path is established to restore the network
connectivity.
However, the newly computed configuration BPDU will not be propagated
throughout the network immediately, so the old root ports and designated ports
that have not detected the topology change continue forwarding data through
the old path. If the new root port and designated port begin to forward data as
soon as they are elected, a temporary loop may occur. For this reason, STP uses a
state transition mechanism. Namely, a newly elected root port or designated port
requires twice the forward delay time before transitioning to the forwarding state,
when the new configuration BPDU has been propagated throughout the network.